Surge brake actuator

Abstract

A hydraulic actuator which may be used as a surge brake actuator may include a housing defining a low pressure chamber and a high pressure chamber, a low pressure piston assembly which is in the low pressure chamber and has a contact surface, a high pressure piston assembly which is in the high pressure chamber and which moves forward in response to forward movement of the low pressure piston assembly, and an unloader valve in fluid communication with the low pressure chamber and having a closed position and an open position adapted to allow hydraulic fluid to flow out of the low pressure chamber through the unloader valve, wherein the contact surface engages and pushes the unloader valve from the closed position to the open position during forward movement of the low pressure piston assembly. The actuator may be used for actuating a brake system.

The invention claimed is: 
     
       1. An actuator comprising:
 a housing defining a low pressure chamber and a high pressure chamber; 
 a low pressure piston assembly which is in the low pressure chamber and has a contact surface; 
 a high pressure piston assembly which is in the high pressure chamber and which moves jointly with the low pressure piston assembly and forward along a longitudinal axis in response to forward movement of the low pressure piston assembly; and 
 an unloader valve in fluid communication with the low pressure chamber and having an elongated stem offset and parallel to the longitudinal axis that moves between a closed position and an open position which allows hydraulic braking liquid to flow out of the low pressure chamber through the unloader valve; 
 wherein the contact surface engages and pushes the elongated stem of the unloader valve longitudinally from the closed position to the open position during forward movement of the low pressure piston assembly along the longitudinal axis so that hydraulic braking liquid flows longitudinally out of the low pressure chamber through the unloader valve. 
 
     
     
       2. The actuator of  claim 1  wherein the unloader valve serves as a pressure relief valve which opens in response to hydraulic braking liquid in the low pressure chamber exceeding a threshold pressure value, and further comprising:
 a rear terminal end on the elongated stem of the unloader valve positioned rearward from a rear end of the high pressure piston assembly. 
 
     
     
       3. The actuator of  claim 1  wherein the the elongated stem forms a portion of a plunger and the unloader valve further includes a housing defining an interior chamber in which the plunger is received; wherein the contact surface engages and pushes a rear terminal end of the stem to move the plunger longitudinally forward from the closed position to the open position during forward movement of the low pressure piston assembly along the longitudinal axis. 
     
     
       4. The actuator of  claim 1  wherein the housing comprises a sidewall and a front wall extending radially inwardly from the sidewall; the sidewall and front wall at least partially define the low pressure chamber; and the front wall defines a longitudinally aligned through hole in which the unloader valve is mounted. 
     
     
       5. The actuator of  claim 1  wherein the low pressure piston assembly includes a seal connected to a forward end of a low pressure piston, wherein the seal is movable inside the high pressure chamber. 
     
     
       6. The actuator of  claim 5  wherein the seal is inside the high pressure chamber when the contact surface engages a rear terminal end of the stem of the unloader valve and the seal is outside the high pressure chamber when the contact surface is disengaged from the rear terminal end of the stem of the unloader valve. 
     
     
       7. The actuator of  claim 5  wherein the low pressure piston assembly has an activated position in which the portion is inside the high pressure chamber and a home position in which the portion is outside the high pressure chamber. 
     
     
       8. The actuator of  claim 5  wherein the low pressure piston assembly has a front section which includes the portion, a flange which extends radially outwardly from the front section and a seal which is forward of and adjacent the flange; and the flange and seal are rearward of the portion. 
     
     
       9. The actuator of  claim 1  wherein the low pressure piston assembly has a forward facing surface; the high pressure piston assembly has a rearward facing surface; and the low pressure piston assembly has an activated position in which the forward facing surface contacts the rearward facing surface; and wherein the rearward facing surface of the high pressure piston assembly is positioned forwardly from a rear terminal end on the elongated stem. 
     
     
       10. The actuator of  claim 1  wherein the low pressure piston assembly has a front end; and the high pressure piston assembly has a rear end; and further comprising a seal extending between the front end and the rear end, wherein the seal moves within the high pressure chamber forwardly along the longitudinal axis when a low pressure piston and a high pressure piston join together and move forward along the longitudinal axis. 
     
     
       11. The actuator of  claim 1  wherein the high pressure piston assembly defines a through hole having front and rear entrance openings; and the front entrance opening is within the high pressure chamber, and the high pressure piston assembly further comprising a seal extending around a high pressure piston, the seal having a radially inward first segment and a radially outward second segment which is secured to an outer radial end of the first segment configured to flare radially outward if the hydraulic braking liquid pressure forward of the seal is higher than the hydraulic braking liquid pressure behind the seal. 
     
     
       12. The actuator of  claim 11  wherein the seal is adjacent the rear entrance opening. 
     
     
       13. The actuator of  claim 1  wherein the low pressure chamber has a front end; and the high pressure chamber extends forward beyond the front end of the low pressure chamber. 
     
     
       14. The actuator of  claim 1  wherein the low pressure piston assembly has a low pressure piston home position in which the low pressure piston assembly is not in contact with the high pressure piston assembly. 
     
     
       15. The actuator of  claim 14  wherein the low pressure piston assembly has a low pressure piston activated position which is forward of the home position and in which the low pressure piston assembly is in contact with the high pressure piston assembly. 
     
     
       16. The actuator of  claim 15  wherein the high pressure piston assembly defines a longitudinally aligned through hole; wherein the longitudinally aligned through hole is in fluid communication with the low pressure chamber when the low pressure piston assembly is in the home position; and the low pressure piston assembly closes the through hole in the activated position so that the longitudinally aligned through hole is not in fluid communication with the low pressure chamber. 
     
     
       17. The actuator of  claim 1  wherein the high pressure piston assembly has a high pressure piston home position and a high pressure piston activated position which is forward of the high pressure piston home position; and the low and high pressure piston assemblies are movable forward together so that the low pressure piston assembly is in contact with the high pressure piston assembly in the high pressure piston home position and in the high pressure piston activated position. 
     
     
       18. The actuator of  claim 1  further comprising a first spring which biases the high pressure piston assembly rearwardly; and a second spring which biases the low pressure piston assembly rearwardly. 
     
     
       19. The actuator of  claim 1  further comprising a first spring which biases the high pressure piston assembly rearwardly; wherein the first spring is in the high pressure chamber and has a rear end which engages the high pressure piston assembly. 
     
     
       20. The actuator of  claim 19  further comprising a second spring which biases the low pressure piston assembly rearwardly; wherein the second spring is in the low pressure chamber and has a rear end which engages the low pressure piston assembly and a front end; and the first spring has a front end which is forward of the front end of the second spring. 
     
     
       21. A method of operating a surge brake actuator comprising the steps of:
 providing a low pressure piston assembly in a low pressure piston chamber and a high pressure piston assembly in a high pressure piston chamber in fluid communication with the low pressure piston chamber; and 
 moving the low pressure piston assembly forward along a longitudinal axis within the low pressure chamber a first distance toward the high pressure piston assembly so that the low pressure piston assembly forces hydraulic braking liquid at a first pressure from the low pressure chamber through a passage in the high pressure piston assembly until the low pressure piston assembly sealingly contacts the high pressure piston assembly to stop additional flow of hydraulic braking liquid through the passage; 
 pushing an unloader valve parallel to the longitudinal axis to open via contact of the low pressure piston assembly with the unloader valve during forward movement of the low pressure piston assembly to allow hydraulic braking liquid to flow out of the low pressure chamber through the unloader valve; and 
 while the unloader valve remains open, moving the low pressure piston assembly forward within the lower pressure chamber a second additional distance so that the low pressure piston assembly pushes the high pressure piston assembly forward whereby the high pressure piston assembly forces hydraulic braking liquid forward of the high pressure piston assembly at a second higher pressure out of the high pressure chamber via a high pressure chamber exit port. 
 
     
     
       22. The actuator of  claim 1  wherein during forward movement of the low pressure piston assembly, the contact surface of the low pressure piston assembly initially contacts the unloader valve when the low pressure piston assembly initially contacts the high pressure piston assembly so that forward movement of the unloader valve and forward movement of the high pressure valve assembly are initiated together. 
     
     
       23. The method of  claim 21  wherein during forward movement of the low pressure piston, the low pressure piston assembly initially contacts the unloader valve when the low pressure piston assembly initially contacts the high pressure piston assembly so that forward movement of the unloader valve and forward movement of the high pressure valve assembly are initiated together. 
     
     
       24. An actuator comprising:
 a housing defining a low pressure chamber and a high pressure chamber; 
 a low pressure piston assembly which is in the low pressure chamber and has a contact surface; 
 a high pressure piston assembly which is in the high pressure chamber and which moves forward along a longitudinal axis in response to forward movement of the low pressure piston assembly; and 
 an unloader valve in fluid communication with the low pressure chamber and having a closed position and an open position which allows hydraulic braking liquid to flow out of the low pressure chamber through the unloader valve, wherein the unloader valve comprises a plunger terminating in a free end positioned rearwardly from the high pressure piston assembly such that the contact surface directly contacts the free end and pushes the plunger forward prior to the low pressure piston assembly, wherein the plunger is offset and aligned parallel to the longitudinal axis; 
 wherein the contact surface pushes the unloader valve from the closed position to the open position during forward movement of the low pressure piston assembly so that hydraulic braking liquid flows out of the low pressure chamber through the unloader valve. 
 
     
     
       25. The actuator of  claim 24 , further comprising:
 a first axis along which the high pressure and low pressure piston assemblies linearly travel; and 
 a second axis associated with the plunger on the unloader valve along which the unloader valve moves from the closed position to the open position, wherein the second axis is offset and parallel to the first axis. 
 
     
     
       26. The actuator of  claim 24 , in combination with a trailer having hydraulically actuated brakes towed by a towing vehicle and the actuator is mounted on the trailer rearwardly from the towing vehicle, wherein the combination comprises:
 a receiving chamber defined by the low pressure piston assembly receiving a pushrod operatively connected to the trailer; 
 wherein the pushrod on the trailer is moved forwardly relative to deceleration of the towing vehicle when the trailer is hitched thereto, and the forward movement of the pushrod moves the low pressure piston assembly forward to impart a braking force to the trailer brakes. 
 
     
     
       27. The actuator of  claim 24  wherein the housing comprises a sidewall and a front wall extending radially inwardly from the sidewall; the sidewall and front wall at least partially define the low pressure chamber; and the front wall defines a through hole in which the unloader valve is mounted offset parallel to the high pressure chamber. 
     
     
       28. The actuator of  claim 24 , wherein the high pressure chamber is isolated and independent such that no braking liquid flows through the unloader valve from the high pressure chamber when a forward end of the low pressure piston assembly sealingly engages the high pressure piston assembly. 
     
     
       29. The actuator of  claim 24 , further comprising:
 a braking application force applied to the low pressure piston assembly wherein the actuator maintains a substantially constant pressure inside an unloader valve chamber when the unloader valve is moved from the closed position to the open position.